The Effect of Changing Conditions on Equilibria
This lesson covers:
- How temperature changes shift the position of chemical equilibrium
- The effect of temperature on the equilibrium constant (Kc)
- Why changes in concentration and pressure do not alter Kc
Increasing temperature favours the endothermic reaction direction
The position of equilibrium depends on temperature according to Le Chatelier's principle:
- For an exothermic reaction, increasing temperature shifts the equilibrium towards the reactants (left)
- For an endothermic reaction, increasing temperature shifts the equilibrium towards the products (right)
This adjustment happens so that the system can counteract the change in temperature.
Example:
N2(g) + 3H2(g) ⇌ 2NH3(g), ΔH = -46.2 kJ mol-1 (exothermic reaction)
If temperature is raised, the equilibrium position will shift to the left to absorb heat.
Temperature changes affect Kc
The equilibrium constant (Kc) is influenced solely by temperature. Changes in concentration or pressure have no impact on Kc.
When the position of equilibrium is altered due to a temperature shift:
- A decrease in the amount of product at equilibrium causes Kc to decrease.
- An increase in the amount of product at equilibrium causes Kc to increase.
This ensures that the value of Kc aligns with the new position of equilibrium.
Example:
2SO2(g) + O2(g) ⇌ 2SO3(g), ΔH = -197 kJ mol-1
Raising the temperature causes the equilibrium to shift left towards the reactants, absorbing heat and resulting in less SO3 at the new equilibrium position.
With a decrease in SO3:
- The proportion of products reduces.
- Consequently, Kc (the ratio of products to reactants) decreases.
The impact of temperature changes on exothermic and endothermic reactions is summarised in the table below:
| Type of reaction | Temperature change | Impact on equilibrium position | Impact on Kc |
|---|---|---|---|
| Exothermic reaction | Increase | Shifts left | Decreases |
| Exothermic reaction | Decrease | Shifts right | Increases |
| Endothermic reaction | Increase | Shifts right | Increases |
| Endothermic reaction | Decrease | Shifts left | Decreases |
Concentration and pressure changes do not affect Kc
While shifts in concentration or pressure can move the equilibrium position, altering the quantities present to counteract the change, Kc remains unchanged at a specific temperature.
For example:
N2(g) + 3H2(g) ⇌ 2NH3(g)
- Adding more N2 leads to an increased formation of NH3 at the new equilibrium. However, the equilibrium ratio of [NH3]2 to [N2][H2]3 remains equal to the constant value of Kc.
- Thus, changes in concentration and pressure do not directly influence Kc - its value is only determined by temperature.
Catalysts do not change Kc or equilibrium position
Catalysts increase the rate of both the forward and reverse reactions, reducing the time needed to reach equilibrium following a change.
However, catalysts neither alter the equilibrium position nor the value of Kc.
The table below summarises the effects of changing reaction conditions on the position of equilibrium and the value of Kc:
| Factor changed | Impact on equilibrium position | Impact on Kc |
|---|---|---|
| Concentration | Changes | Remains the same |
| Pressure | Changes | Remains the same |
| Temperature | Changes | Changes |
| Catalyst | Remains the same | Remains the same |